minor #19426 [Serializer] add note in DiscriminatorMap section (Enz000)

This PR was submitted for the 7.0 branch but it was merged into the 6.3 branch instead.

Discussion
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[Serializer] add note in DiscriminatorMap section

Regarding this [issue](symfony/symfony#53387) I added a note about the discriminator map

Commits
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05b6ce4 add note in DiscriminatorMap section

Author: javiereguiluz

components/serializer.rst

@@ -1837,6 +1837,11 @@ and ``BitBucketCodeRepository`` classes:
             </class>
         </serializer>
 
+.. note::
+
+    The values of the array parameter `mapping` should be strings.
+    Otherwise, it will implicitly be cast into strings.
+
 Once configured, the serializer uses the mapping to pick the correct class::
 
     $serialized = $serializer->serialize(new GitHubCodeRepository(), 'json');